Ultrafast dephasing of surface plasmon excitation in silver nanoparticles: influence of particle size,shape, and chemical surrounding

By combination of two special methods, i.e., persistent spectral hole burning and laser assisted nanoparticle preparation, the dephasing time T2 of surface plasmon excitation in silver nanoparticles was systematically investigated. A strong dependence of T2 on the plasmon energy is found which reflects the relevance of interband damping and makes necessary a precise control of the particle shape when measuring T2. The influence of the reduced dimension on the dephasing dynamics was observed as a decrease of T2 with shrinking particle size. In addition, for silver nanoparticles on quartz substrates, a considerable amount of chemical interface damping was observed.     

Driving-dependent damping of Rabi oscillations in two-level semiconductor systems

We propose a mechanism to explain the nature of the damping of Rabi oscillations with an increasing driving-pulse area in localized semiconductor systems and have suggested a general approach which describes a coherently driven two-level system interacting with a dephasing reservoir. Present calculations show that the non-Markovian character of the reservoir leads to the dependence of the dephasing rate on the driving-field intensity, as observed experimentally. Moreover, we have shown that the damping of Rabi oscillations might occur as a result of different dephasing mechanisms for both stationary and nonstationary effects due to coupling to the environment. Present calculated results are found in quite good agreement with available experimental measurements.     

Charge‐Induced Shifts in Chiral Surface Plasmon Modes in Gold Nanorod Assemblies

A detailed investigation of the effect of electron injection on the chiral plasmon modes of helical nanorod assemblies is presented. An increased surface plasmon frequency of the electron gas due to the addition of electrons leads to a blue‐shift in the corresponding chiral surface plasmon modes. The mechanism behind the shift in plasmonic chirality due to nanorod charging is investigated using theoretical simulations. Charging of the nanorods alters the surface electron density, thereby modifying the plasma frequency and causing a change in the dielectric function. The nature of the plasmon shift and the intensity of chiral surface plasmons are found to be largely dependent on the extent of electron addition. At extended periods of time, the blue shifted band slowly shifts back toward the red, due to transfer of electrons back to the medium, leading to discharging of the nanorods.     

Damping of the localized surface plasmon polariton resonance of gold nanoparticles

We report on a strong damping of the localized surface plasmon polariton resonance of gold nanoparticles. The ultra-fast dephasing time of localized surface plasmon polariton resonances in gold nanoparticles was systematically studied as a function of the particle size at a fixed photon energy of h ν=1.85 eV. Dephasing times ranging from T₂^expT_{2}^{\mathrm{exp}} = 5.5 fs to 15.0 fs were extracted and an influence of the reduced dimensions was detected. We have identified two dominant damping mechanisms: the well-known surface scattering and, for the first time, band structure changes. We have quantified the influence of these band structure changes on the optical properties by determining the essential damping parameter A to be A ^exp=0.32 nm/fs.     

金纳米棒三聚体中的等离激元诱导透明

基于金纳米棒构成的三聚体微元结构模型,详细地研究了等离激元诱导透明(plasmon induced transparency,PIT)现象产生的物理过程.研究发现,三聚体的吸收谱线随着其耦合距离以及尺寸的变化,竖直金纳米棒所对应的偶极明模在平行双长条金纳米棒对应的暗模作用下会产生分裂.依据这一结果提出了一个新的物理解释,PIT现象的产生主要来自于竖直金纳米棒中偶极振荡的模式分裂后的相干叠加.同时,考虑到两个振子之间的耦合会伴随着一定的相位关联性,进而引入了耦合相位因子修正了洛伦兹振子耦合模型,解析地研究了耦合相位因子对吸收谱的调控作用和分裂明模之间的相干叠加效应对PIT效应的影响.这为在纳米尺寸范围设计人造原子、光开关、慢光效应等方面的应用提供了理论参考.     

Intrinsic damping for ultrafast laser-excited acoustic vibrations of single gold nanorods

The intrinsic damping for the acoustic vibrations of single gold nanorods excited by ultrafast laser has been studied through the atomistic simulations. It is shown that the intrinsic damping for the breathing mode is strongly sensitive to the nanorod sizes, which is very likely due to the different energy redistributions between the vibrational modes of nanorods and could play a non-negligible role in the broad distribution of the experimentally measured breathing-mode quality factors. In comparison, the intrinsic damping for the extensional vibration of gold nanorods appears much less influenced by the variations of nanorod dimensions. Moreover, we also find that the intrinsic mechanism is a significant source for the vibrational damping of gold nanorods, particularly for the breathing mode.     

Optical Bloch Equations Modified with Phonon-Induced Intensity-Dependent Dephasing

We extend the exciton population equations of a two-level quantum dot system with weak excitation to the ones with strong excitations, in which, the phonon-induced intensity-dependent dephasing time and decay rate are involved. The straightforward calculated populations from the modified population equations demonstrate the damping behavior of Rabi oscillation as the external field increasing. The effect of the intensity-dependent dephasing time and the intensity-dependent decay rate are also discussed.     

Optical Bloch Equations Modified with Phonon-Induced Intensity-Dependent Dephasing

We extend the exciton population equations of a two-level quantum dot system with weak excitation to the ones with strong excitations, in which, the phonon-induced intensity-dependent dephasing time and decay rate are involved. The straightforward calculated populations from the modified population equations demonstrate the damping behavior of Rabi oscillation as the external field increasing. The effect of the intensity-dependent dephasing time and the intensity-dependent decay rate are also discussed.     

Geometric phase of an open double-quantum-dot system detected by a quantum point contact

We study theoretically the geometric phase of a double-quantum-dot(DQD) system measured by a quantum point contact(QPC) in the pure dephasing and dissipative environments, respectively. The results show that in these two environments, the coupling strength between the quantum dots has an enhanced impact on the geometric phase during a quasiperiod. This is due to the fact that the expansion of the width of the tunneling channel connecting the two quantum dots accelerates the oscillations of the electron between the quantum dots and makes the length of the evolution path longer.In addition, there is a notable near-zero region in the geometric phase because the stronger coupling between the system and the QPC freezes the electron in one quantum dot and the solid angle enclosed by the evolution path is approximately zero,which is associated with the quantum Zeno effect. For the pure dephasing environment, the geometric phase is suppressed as the dephasing rate increases which is caused only by the phase damping of the system. In the dissipative environment,the geometric phase is reduced with the increase of the relaxation rate which results from both the energy dissipation and phase damping of the system. Our results are helpful for using the geometric phase to construct the fault-tolerant quantum devices based on quantum dot systems in quantum information.     

Plasmon modes in double-layer gapped graphene at zero temperature

Plasmon dispersions and damping rate of plasma oscillations in a double-layer gapped graphene made of two parallel mono layer gapped graphene sheets grown on dielectric separation are calculated within random-phase-approximation at zero temperature. By using long wavelength limit expansion, analytical expressions for optical and acoustic plasmon frequencies have been formed, and the formulae demonstrate that the considerable difference in analytical form for plasmon frequencies comes from the factor depending on the band gap, compared to gapless situation. Numerical results show that only large band gap decreases remarkably plasmon frequencies of two modes in the range of large wave vector. Acoustic plasmon branch becomes shorter than that in case of zero gap while optical one seems independent with small band gap. In addition, interlayer separation and carrier density affect on collective excitations and damping rate when taking into account the band gap quite similarly to those in case of zero gap.     

Chemical interface damping of surface plasmon excitation in metal nanoparticles: a study by persistent spectral hole burning

As demonstrated recently, persistent spectral hole burning in the optical spectra of metal nanoparticles makes possible the determination of the ultrafast dephasing time T₂ of surface plasmon excitation. Here, the influence of the chemical environment on the dephasing process is investigated under ultrahigh vacuum conditions by depositing an adsorbate on the nanoparticles. By comparing the optical spectra and the widths of the generated holes of the nanoparticles with and without adsorbate coverage, the influence of chemical interface damping on the dephasing process is examined. The potential of the novel procedure is demonstrated for silver nanoparticles on sapphire and quartz substrates using SO₂ as the adsorbate. A drastic decrease of T₂ for the adsorbate-covered nanoparticles is observed and explained by dynamic charge transfer of electrons from the particles into and out of adsorbate states of the SO₂ molecules. PACS 78.67.Bf; 61.46.+w; 71.45.Gm; 73.22.Lp; 68.43.-h     

Temperature effect on plasmon dispersions in double-layer graphene systems

We investigate the plasmon dispersion relation and damping rate of a double-layer graphene system consisting of two separated monolayer graphenes with no interlayer tunneling at finite temperature. We use the temperature dependent RPA dielectric function which is valid for graphene systems to obtain the plasmon frequencies and damping rates at different temperatures, interlayer correlation parameters and electron densities and then compare them with those obtained from the zero temperature calculations. Our results show that by increasing the temperature, the plasmon frequencies decrease and the decay rate increases. Furthermore, we find that the behavior of a double-layer graphene system at small and large correlation parameters is different from the conventional double-layer two-dimensional electron gas system. Finally, we obtain that in a density imbalanced double-layer graphene system, the acoustic plasmons are more affected by temperature than the equal electron densities one.     

Preparation of monodisperse bimetallic nanorods with gold nanorod core and silver shell and their plasmonic property and SERS efficiency

In this study, monodisperse bimetallic nanorods with gold (Au) nanorod core and silver (Ag) shell (Au@AgNRs) were synthesized through seed‐mediated growth process by reduction of AgNO₃ using Au nanorods with narrow size and shape distribution as seeds. With increasing the used amount of AgNO₃, the Ag shell thickness of their lateral facets is raised faster than that of their two tips, leading to a decrease of their aspect ratios. Four plasmon bands are observable on the extinction spectra of Au@AgNRs, which are attributed to the longitudinal dipolar plasmon mode, transverse dipolar plasmon mode, and octupolar plasmon mode of the core‐shell structured bimetallic nanorods, respectively. As their Ag shell thickness increases, their longitudinal plasmon band blue‐shifts notably with the transverse plasmon band blue‐shifting and the two octupolar plasmon bands red‐shifting slightly, due to the decrease of their aspect ratios and enhancement of Ag plasmon resonance contribution. When used as surface‐enhanced Raman scattering (SERS) substrate for probing minute amounts of 4‐mercaptobenzoic acid in aqueous solution, Au@AgNRs have much stronger SERS activity than Au nanorods, and the obtained Raman signals are highly reproducible arising from their excellent monodispersity. Their SERS activity is remarkably increased with their Ag shell thickness thanks to the enhancing surface electric field and the chemical enhancement associated with electronic ligand effect. Copyright © 2014 John Wiley & Sons, Ltd.     

Decrease in the damping rate of Rabi oscillations of artificial atoms at nonresonant excitation

A decrease in the damping rate of Rabi oscillations with an increase in the detuning of the frequency of exciting radiation from the resonance frequency of a two-level quantum system has been explained. This effect is implemented when the rate of the pure dephasing of the quantum system is lower than the longitudinal relaxation rate. It can be observed, e.g., for artificial atoms such as semiconductor quantum dots for which pure dephasing processes are almost absent.     

Surface plasmon damping due to rough boundary scattering

The surface plasmon damping due to carriers scattering at the statistically rough (semiconductor-dialectric) interface is considered. The specular parameter and the integral relation are used as the boundary condition for a non-equilibrium part of the distribution function. There exist certain cases when the rough surface scattering of carriers is shown to play an important role in the surface plasmon damping.     

Plasmon hybridization in nanorod dimers

Using the plasmon hybridization method we investigate the plasmon modes of nanorod dimers in axial and parallel orientations. We show that the plasmon modes of the system can be viewed as bonding and anti-bonding modes resulting from the hybridization of the plasmon modes of the individual nanorods. The dimer plasmon modes are found to depend sensitively on separation between the nanorods and on their relative spatial orientation. The calculated optical properties agree quantitatively with results from the numerical finite-difference time-domain method. The electric field enhancements are found to depend strongly on aspect ratio defined as the ratio of the major and minor radii, and on the relative orientation of the nanorods. For a nanorod dimer of fixed overall length, the maximum field enhancements are lower than those induced in a solid sphere dimer.     

Fano resonances in dipole-quadrupole plasmon coupling nanorod dimers

We theoretically investigate the plasmon coupling in metallic nanorod dimers. A pronounced dip is found in the extinction spectrum due to plasmonic Fano resonance, which is induced by destructive interference between the bright dipole plasmon of a short nanorod and the dark quadrupole plasmon of a long nanorod. This Fano interference can also be explained as the coupling between the bright and dark modes both supported by the whole dimer. The Fano resonance can be tuned by adjusting the spatial or spectral separation between two nanorods in the dimer.     

Plasmon modes in MLG-2DEG heterostructures: Temperature effects

We calculate the plasmon frequency and damping rate in a double-layer system made of monolayer graphene and GaAs quantum well at finite temperature using the random-phase-approximation dielectric function and taking into account the inhomogeneity of the dielectric background of the system. We show that the temperature, interlayer correlation parameters and dielectric background inhomogeneity affect significantly the plasmon frequencies and damping rates of the system. At low temperatures, acoustic (optical) plasmon frequency increases (decreases) with the increase of temperature. We also find that damping rates of both plasmon modes increase remarkably compared to the zero-temperature case.     

Ultrafast dephasing of localized surface plasmons in colloidal silver nanoparticles: the influence of stabilizing agents

Localized surface plasmon dephasing times for aqueous colloidal silver nanoparticles (NPs) stabilized with three different capping agents (trisodium citrate??TSC, poly(vinylpyrrolidone)??PVP, and poly(vinylalcohol)?? PVA) were measured using the persistent spectral hole burning technique. The results obtained by fitting a theoretical curve to the experimental data show that the dephasing times are dependent on the chosen stabilizer (3.0, 2.3, and 1.8?fs for TSC, PVP, and PVA, respectively), and the differences are attributed to changes in the electronic density of states due to the interaction between the NPs and the capping agents. The results are supported by ab initio calculations for the chemisorbate and metallic cluster interaction.     

Surface enhanced Raman scattering from mildly roughened surfaces: variation of signal with metal grain size

The intensity of surface enhanced Raman scattering from benzoic acid derivatives on mildly roughened, thermally evaporated Ag films shows a remarkably strong dependence on metal grain size. Large grained (slowly deposited) films give a superior response, by up to a factor of 10, to small grained (quickly deposited) films, with films of intermediate grain size yielding intermediate results. The optical field amplification underlying the enhancement mechanism is due to the excitation of surface plasmon polaritons (SPPs). Since surface roughness characteristics, as determined by STM, remain relatively constant as a function of deposition rate, it is argued that the contrast in Raman scattering is due to differences in elastic grain boundary scattering of SPPs (leading to different degrees of internal SPP damping), rather than differences in the interaction of SPPs with surface inhomogeneities.